Apply toValue conversion on assert messages.

pkg/fasta/lib/src/kernel/body_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.body_builder;
 
 import 'package:dart_parser/src/parser.dart' show
     FormalParameterType,
     optional;
 
@@ skipping 151 matching lines @@
   void push(Object node) {
     isFirstIdentifier = false;
     inInitializer = false;
     super.push(node);
   }
 
   Expression popForValue() => toValue(pop());
 
   Expression popForEffect() => toEffect(pop());
 
+  Expression popForValueIfNotNull(Object value) {
+    return value == null ? null : popForValue();
+  }
+
   @override
   Expression toValue(Object node) {
     if (node is UnresolvedIdentifier) {
       return throwNoSuchMethodError(node.name.name, new Arguments.empty(),
           node.fileOffset, isGetter: true);
     } else if (node is BuilderAccessor) {
       return node.buildSimpleRead();
     } else if (node is TypeVariableBuilder) {
       TypeParameterType type = node.buildTypesWithBuiltArguments(null);
       if (!isInstanceContext && type.parameter.parent is Class) {
@@ skipping 1722 matching lines @@
   @override
   void handleEmptyStatement(Token token) {
     debugEvent("EmptyStatement");
     push(new EmptyStatement());
   }
 
   @override
   void handleAssertStatement(
       Token assertKeyword, Token commaToken, Token semicolonToken) {
     debugEvent("AssertStatement");
-    Expression message = popIfNotNull(commaToken);
+    Expression message = popForValueIfNotNull(commaToken);
     Expression condition = popForValue();
     push(new AssertStatement(condition, message));
   }
 
   @override
   void endYieldStatement(Token yieldToken, Token starToken, Token endToken) {
     debugEvent("YieldStatement");
     push(new YieldStatement(popForValue(), isYieldStar: starToken != null));
   }
 
@@ skipping 715 matching lines @@
   } else if (node is TypeDeclarationBuilder) {
     return node.name;
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisPropertyAccessor) {
     return node.name.name;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }